As a method of chemically polymerizing indole, JP-A-5-148320 proposes a method including the step of dripping an indole solution into an oxidant solution. This method assumes only the case where indole containing no substituent is used as the raw material. There is no mention of an indole derivative containing a substituent. In addition, the product is a polymer containing indole as a repeating unit, and the publication does not include any description concerning formation of trimer.
Synthesis of polymer by means of an electrolytic reaction of indole without any substituent, 4-nitroindole or 5-nitroindole is described in J. Electroanal. Chem., 414 (1996), p. 197 and synthesis of a trimer by means of an electrolytic reaction of indole without any substituent or 5-cyanindole is described in J. Chem. Soc., Faraday Trans., 93 (1997), p. 3791, respectively. When an oxidation and reduction testing (cycle testing) is performed using the unsubstituted or substituted indole produced by the method described in the documents, degradation of the trimer occurs, and as a result the cycle characteristics is rather poor, and the products can hardly be applied for production of conductive devices. Further, it is impossible to synthesize trimers on a large scale by the electrolytic reaction, and the method can hardly be employed as an industrial process.
Synthesis of unsubstituted trimer using TiCl3 or FeSo4 and H2O2 is reported in Heterocycles, 12 (1979), p. 471, and in Chem. Pharm. Bull., 29, (1981), p. 3499. As described in the articles, however, there are many disadvantages, for instance, that several types of byproducts are mixed in the product, and that the yield of the target trimer is rather low.
Further, synthesis of unsubstituted indole trimer by means of the electrolytic reaction is disclosed also in Synthetic Metals, 80 (1996), p. 309. The physical properties of the trimer are: the interlaminar spacing of 0.658 nm and conductivity of 0.03 S/cm.
As described above, the unsubstituted and substituted indole trimers produced by the conventional technology have the insufficient cycle characteristics and low adaptability to industrial production. Therefore, the development of indole derivative trimers having a high oxidation—reduction potential, a high oxidation—reduction capacity, and excellent cycle characteristics, as well as the development of a method of industrially producing the indole derivative trimers have been very important problems to be solved in the art.
It is an object of the present invention to provide a method capable of industrially producing indole derivative trimers with high purity as well as to provide novel indole derivative trimers having a high conductivity, a high oxidation—reduction potential, a high oxidation—reduction capacity, and excellent cycle characteristics, obtainable by the method.